Fix home inbox header blur

[thomaspblock]

Summary

• Add a single continuous backdrop-blur layer across the Home inbox grid header.
• Make the list/detail pane header rows transparent so the shared blur blends across the column boundary instead of clipping per pane.
• Add Playwright coverage that verifies the shared grid pseudo-element owns the blur while pane headers remain transparent.

Test plan

• pnpm --dir desktop check
• pnpm --dir desktop build
• pnpm --dir desktop exec playwright test --project=smoke tests/e2e/home-collapsed-top-chrome.spec.ts
• just desktop-screenshot --name inbox-header-blur-final --route / --clip 250,0,1030,140 --wait 1500